Coolable electric cable

ABSTRACT

A coolable cable such as a cooled superconductive cable or the like has one or more electric conductors and includes a conduit concentric therewith for directing a coolant therealong. A plurality of tubes are provided in parallel spaced relation to the electric conductor and a metal band is wound around the tubes and forms a radiation shield. Supporting members are disposed intermediate the conductor and the tubes at respective positions along the longitudinal cable axis for supporting the tubes and the metal band.

Patented April 18, 1972 COPPER MATRIX 3 COOLABLE ELECTRIC CABLE Ourinvention relates to a cooled, preferably superconducting cable having aradiation shield of metal surrounding the electrical conductor orconductors coolable by means of a first cooling means. The shield isjoined to several tubes which serve to direct a second cooling means.

Considerable advantages are obtainable from superconducting cables inthe transmission of large amounts of electrical energy. For thesecables, electrically normal conducting metals of especially high puritycan be used as electrical conductors such as highly pure aluminum. Sincethe ohmic resistance of nonnal conducting metals is substantiallysmaller at low temperature than at room temperature, the electricallosses in the cable can be greatly reduced by cooling the conductors.Especially suitable for this type of cable are superconducting metalswhose ohmic resistance completely disappears with cooling to atemperature beneath the critical or transition temperature of thesuperconducting material used. In this connection, as superconductingmaterials, the metals niobium and lead as well as the so-calledhigh-field superconductive materials are especially to be considered.Examples of high-field superconductive materials are superconductingalloys of niobium and titanium or of niobium and zirconium, ifnecessary, these alloys are provided with additions of furthermaterials. Another example which can be mentioned are intermetalliccompounds of niobium-tin (Nb Sn). The superconductors can be joined ingood electrically conductive and good heat conductive relation to metalssuch as copper or aluminum for electrical stabilization, these metalsbeing connected electrically in parallel with the superconductors andbeing electrically normal conducting at the operating temperature of thesuperconductor. Or, the superconductors can be imbedded in theseelectrically normal conductive metals.

' To cool conductors of electrically normal conducting metal, fluidsespecially suited are those having a boiling temperature of underapproximately 150 K such as liquid hydrogen, liquid nitrogen or naturalgas or cold gases of corresponding temperatures. For coolingsuperconductors, with the superconductive materials available at thepresent time, practically only liquid or cold helium in the gaseousstate can be considered.

To keep the cooling capacity needed to cool the conductors to thesmallest amount possible and to prevent unnecessary losses of coolingmedium, the conductors and the cooling medium for cooling the conductorsmust be thermally insulated with respect to the exterior region of thecable. As a rule, this thermal insulation consists of an evacuated spacesurrounding the conductors. A reduction of the heat radiation impingingupon the conductors from the region surrounding the cable can beachieved by providing a radiation shield of sheet metal surrounding theconductors and cooled by means of a second cooling means having atemperature higher than the cooling means for cooling the conductors.According to a paper of E. C. Rogers and D. R. Edwards in the journalElectrical Review 181 (l967), P. 348 to 351, with superconducting cablescooled with liquid helium, this radiation shield consists of a sheetmetal tube joined with several tubes.

The foregoing type of radiation shields of sheet metal have severaldisadvantages. For example, they must be preassembled before the cableis layed and transported to the location where the cable is to be layed.This can contribute significantly to the transport difficulties becauseof the relatively large diameter of the tubular or pipe-like radiationshield. In addition, because of the various movements of the conductorsof the cable and the pipe-like radiation shield associated with areduction of temperature to different levels, provisions must be made tocancel out the different changes in length. For this purpose, forexample, expansion bellows can be required between individual parts ofthe pipe-like radiation shield.

It is an object of our invention to provide a cooled cable whichovercomes the aforementioned difiiculties associated with coolablecables.

More particularly, it is an object of our invention to provide a cooled,preferably superconducting cable having a radiation shield that has asimplified construction and improved operating characteristics.

According to a feature of the invention, theradiation shield comprises ametal band wrapped around tubes serving to direct the second coolingmeans and, if required, around additional supporting members.

Such a metal band can be moved to the location whereat the cable is tobe layed without transporting difficulties. in addition, practically nodifiiculties are encountered because of a pulling together of theradiation shield with cooling because the metal band can be woundsomewhat loose, so that it tenses with a shrinkage caused by cooling andbecause, in addition, the individual windings of the metal band candisplace somewhat against each other in a direction along thelongitudinal axis of the cable.

Materials suitable for the metal band are preferably good heatconductive metals such as copper or aluminum. Because of costconsiderations the likewise suitable metals of gold and silver are notpractical. The mentioned metals have a high reflectivity in theinfra-red region decisive for the impinging heat radiation. Since theemission of infra-red radiation of a body is the same as the absorbtionthereof, the higher the reflectivity, the more effective is theoperation of the radiation shield. This arrangement affords the specialadvantage that the metal bands can be produced with considerably bettersurface qualities than metal tubes, that is, with a smoother surface,and that the reflectivity of the radiation shield made of a metal bandis therefore greater and the emission capability for infra-red radiationis smaller than with a radiation shield made of sheet metal. Stainlesssteel is a material useable for the metal band.

As additional supporting members for the radiation shield, it isadvantageous to provide tube-shaped channels supported by spoke-likesupports of poor heat conductive material braced against a pipesurrounding the electric conductor or conductors. The channels areadvantageously joined to the tubes serving to carry the second coolingmeans with metal bridging members. The bridging members can beconstructed so that they form a ring joining the channels with the tubesserving to guide or direct the second cooling means. In this manner,with respect to the electrical conductor or conductors, an especiallystabile holder of good heat isolating material is provided for theradiation shield. The spoke-like supports are preferably slidably bornein the channels. With such a bearing arrangement, the channels canslidably move with respect to the spoke-like supports in the directionof the longitudinal cable axis, so. that the different contractions ofthe radiation shield and the tube surrounding the conductors occurringwith cooling can cancel.

In order to obtain the best possible thermal contact between the metalband and the tubes serving to direct the second cooling means, the metalband is advantageously pressed onto the tubes with cylinders or rollers,or is squeezed with the tubes when the radiation shield is formed. Thesurfaces of the tubes and the band are intermeshed with each other as aresult of the squeezing action. For further improving the thermalcontact, the band can also be welded with the tubes for directing thesecond cooling means.

The invention will now be described with reference to the accompanyingdrawing wherein:

FIG. 1 is a schematic diagram, in section, of a cooled cable of theinvention provided with a radiation shield.

FIG. 2 is a side view of the radiation shield of the cable of FIG. 1.

FIGS. 3a to 3c illustrate several arrangements of welding the metal bandof the radiation shield with the tubes serving to direct or guide thesecond cooling means.

Referring to the cooled cable of FIG. 1, the electrical conductor l isarranged within a pipe or conduit 2 made, for example, of stainlesssteel and serving to direct the first cooling means. The conductor 1 canbe, for example, a copper matrix 3 in which a plurality ofsuperconductive niobium-titanium wires 4 can be imbedded. To coolconductor 1, liquid helium can be used which during the operation of thecable streams through the interior space 5 of pipe 2. The pipe 2 issurrounded by another vacuum tight pipe 6 which can likewise be made ofstainless steel. The free space 7 between the two pipes 2 and 6 isevacuated to thermally insulate the cooled conductor 1 from thesurrounding environment of the cable. Within this space 7, a radiationshield 8 comprising a copper band is provided. The copper band is woundabout three tubes 9 and three additional tubular channels 10, thechannels 10 serving as supporting members. The tubes 9 consist, forexample, of stainless steel. The tubes 9 serve to direct the secondcooling means for cooling the radiation shield. During operation of thecable, liquid nitrogen, for example, streams through the tubes 9. Thetubular channels 10 which serve as supporting members, can beadvantageously made of a poor heat conductive plastic or of ceramic. Thetubes 9 and the channels 10 are positioned with respect to each other bymeans of metal rings to which they are admitted. One such ring 11 isillustrated in FIG. I and other such rings are disposed at positionsalong the longitudinal axis of the cable. The ring 11 can be constructedso as to be separable into parts, so that during the assembly of thecable, the rings 11 do not have to be slid onto the pipe 2. Instead, therings 11 can be placed about the pipe 2 at their respective locationsalong the cable. Spoke-like supports 12 of poor heat conductivematerial, for example plastic or ceramic, are slidably borne by thetubular channels 10. The spoke-like supports 12 are braced against thepipe 2 and are held in place on this pipe by ring 13 made of poorheat-conductive material. The rings 13 can, for example, be constructedso as to consist of separable parts. Additional spoke-like supports 12and rings 13 of the aforementioned types are disposed at positions alongthe longitudinal axis of the cable. The supports 12 and rings 13 arepreferably spatially displaced toward the metal ring 11, so that asliding of the spoke-like supports 12 in the tubular channels 10 is notprevented by the ring 11. Channels 10 made of plastic or ceramic affordthe advantage that between the radiation shield 8 and the pipe 2 only asmall amount of heat passes. In addition, these materials make itpossible for the spoke-like support elements to slide easily. For anadditional improvement in the heat insulation, layers of poorheat-conductive foils 14 can be provided intermediate the radiationshield 8 and the pipe 6. Especially suitable for this purpose are layersof polyethyleneterephtalate foils which are preferably coated withreflecting aluminum.

FIG. 2 is a side view of the radiation shield of the cable of FIG. I andillustrates how the copper band 15 forms the radiation shield withoverlapping edges. If desired, several bands can be wound a plurality ofturns next to each other.

In FIGS. 3a to 30 are illustrated several possibilities as to how thecopper band 15 can be welded to the tubes 9 serving to direct the secondcooling means. It is preferable to form the welded joint between band 15and tubes 9 by means of resistance welding wherewith the weldingelectrodes are disposed at the locations designated by the arrows 16.The welding procedure is advantageously combined with the windingprocedure, so that the finished winding of the band 15 which forms theradiation shield is joined with the tubes 9 by means of a continuousseam weld running in the direction of the longitudinal axis of thesetubes.

With other forms of the cable, in the place of the conductor 1illustrated in FIG. 1, a tubular conductor can be used having an outersurface coated with a layer of superconductive material. Also, the pipe2 can, for example, be coated with a layer of superconductive materialon its inner surface, the layer serving to conduct the current. Also,within the pipe 2, several conductors could be disposed instead of theone conductor l. The conductor 1 could be made of aluminum for examplein a cooled normal conducting cable. As a cooling means for theconductor, liquid hydrogen can be used for example.

Also the support of the radiation shield 8 can be modified in many ways.Especially the number of tubes 9 serving to direct the second coolingmeans and the number of tubular channels 10 can be varied. At the placeof the metal rmg 11 between the tubes 9 and the channels 10, bridge-likestruts can be arranged. In addition, the channels 10 can consist ofmetal if the spoke-like supports 12 are sufficiently heat resistant.

To those skilled in the art it will be obvious upon a study of thisdisclosure that our invention permits of various modifications and maybe given embodiments other than particularly illustrated herein, withoutdeparting from the essential features of the invention and within thescope of the claims annexed hereto.

We claim:

1. A coolable cable comprising electrical conductor means which includesa conduit concentric therewith for directing a cyrogenic coolanttherealong, a plurality of tubes disposed in parallel spaced relation tosaid conductor means for directing the flow of a second cyrogeniccoolant, a metal band wound about said tubes and forming a radiationshield and support means disposed intermediate said conductor means andsaid tubes for supporting the latter and said metal band.

2. In a cable according to claim 1, said band consisting of a metalselected from the group consisting of copper and aluminum.

3. A cable according to claim 1 wherein said conductor means comprisesat least one electrical conductor disposed within said conduit, andwherein said support means has tubular channels disposed in parallelspaced relation with respect to said conduit, and spoke-like members ofpoor heat conductive material disposed intermediate said conduit andsaid channels for bracing the latter with respect to the former.

4. A cable according to claim 3 wherein said channels each have bridgingmembers connected between the mutually adjacent channels thereof forsupporting said plurality of tubes relative to said channels.

5. In a cable according to claim 4, said bridging members forming a ringholding channels and said tubes in place.

6. In a cable according to claim 5, the spokes of said spokelike memberseach having an end slidably mounted in a corresponding one of saidchannels.

7. In a cable according to claim 1, said metal band being press joinedto said plurality of tubes.

8. In a cable according to claim 1, said metal band being clamp joinedto said plurality of tubes.

9. In a cable according to claim 1, said metal band being welded to saidplurality of tubes.

10. A cable according to claim 1, said electrical conductor meanscomprising at least one electrical conductor disposed within saidconduit, said conductor exhibiting superconductive characteristics whensaid first-mentioned cyrogenic coolant is at a temperature below thecritical temperature corresponding to said conductor.

2. In a cable according to claim 1, said band consisting of a metalselected from the group consisting of copper and aluminum.
 3. A cableaccording to claim 1 wherein said conductor means comprises at least oneelectrical conductor disposed within said conduit, and wherein saidsupport means has tubular channels disposed in parallel spaced relationwith respect to said conduit, and spoke-like members of poor heatconductive material disposed intermediate said conduit and said channelsfor bracing the latter with respect to the former.
 4. A cable accordingto claim 3 wherein said channels each have bridging members connectedbetween the mutually adjacent channels thereof for supporting saidplurality of tubes relative to said channels.
 5. In a cable according toclaim 4, said bridging members forming a rIng holding channels and saidtubes in place.
 6. In a cable according to claim 5, the spokes of saidspoke-like members each having an end slidably mounted in acorresponding one of said channels.
 7. In a cable according to claim 1,said metal band being press joined to said plurality of tubes.
 8. In acable according to claim 1, said metal band being clamp joined to saidplurality of tubes.
 9. In a cable according to claim 1, said metal bandbeing welded to said plurality of tubes.
 10. A cable according to claim1, said electrical conductor means comprising at least one electricalconductor disposed within said conduit, said conductor exhibitingsuperconductive characteristics when said first-mentioned cyrogeniccoolant is at a temperature below the critical temperature correspondingto said conductor.